I just made some water-slide decals for the first time on my inkjet printer. There are a few places selling them, some types of the paper are available from hobbytown stores.

Just make the design you want using photoshop, or better yet a vector-based program like CorelDraw or Illustrator (that's probably the hardest part if you're not familiar with those packages, but they produce the cleanest graphics), or if worst comes to worst draw it out, scan it, and use any number of desktop publishing packages to print them out to scale.

Print, and check your artwork on white paper, then just use the special decal paper.

The sheet I got suggested I should use "econo" settings on the printer (not sure why). Other websites said use the "glossy photo paper" settings. I ended up using high-resolution glossy photo paper settings with no problem, let the ink set up for a short time then lightly sprayed 3 light coats of clear on them. They went on fine and look pretty good.